The present invention relates to an adaptive array apparatus that adaptively generates directivity patterns for a plurality of antennas.
To raise transfer efficiency, digital communication devices that transfer information by modulating a carrier wave using a digital information signal (baseband signal) have been developed in recent years.
In digital communication, frequencies can be used more efficiently by raising the transfer speed or by using time division multiplexing to convert single frequencies into multichannels for use by several users. Raising the transfer speed, however, leads to deterioration in quality due to fading.
A number of techniques to counteract this problem have been developed. A representative technique is the adaptive array method. This method adaptively generates directivity patterns (also referred to as xe2x80x9cadaptive array patternsxe2x80x9d) using a plurality of antennas and generates an electromagnetic wave so that they only reach a user in a specified direction. First, consider an adaptive array apparatus that has four radio units that each include a transmission circuit, a reception circuit, and an antenna. Separate directivity patterns for transmission and reception can be generated for each radio unit by adjusting the gain value of each transmission circuit and the phase of each transmission signal during transmission and by adjusting the gain value of each reception circuit and the phase of each reception signal during reception. The adaptive array method is described in detail in Adaptive Signal Processing for Spatial Regions and Its Technical Applications (in xe2x80x9cTransactions of the Institute of Electronics, Information, and Communication Engineers (IEICE) of Japanxe2x80x9d) Vol. J75-B-II No. 11, November 1992.
To perform bidirectional communication using the adaptive array method, it is desirable to have directivity patterns formed by both devices in communication. When this is applied to mobile communication, however, the physical limitations on the size of the mobile devices and the number of antennas used by them make it effectively impossible for mobile devices to form directivity patterns. Accordingly, the base station forms separate directivity patterns for both transmission and reception. This means that during transmission, the base station forms a directivity pattern that is the same as the ideal directivity pattern that was formed during reception and uses it when transmitting signals. In this way, the base station forms different directivity patterns for a plurality of mobile devices and so can multiplex a number of simultaneous connections on a single frequency. This type of communication is called Path Division Multiple Access, or xe2x80x9cPDMAxe2x80x9d for short. PDMA is described in Path Division Multiple Access (PDMA) Mobile Radio Communications Systems (Suzuki, H., and K. Hirade) which was published on pages 37-44 of Technical Report of IEICE, RCS-93-84 January 1994, so that no further explanation will be given here.
Conventional adaptive array apparatuses have a problem in that while it is possible to use one wave (frequency) efficiently by forming different directivity patterns, it has not been possible to do this for a plurality of waves (frequencies). As one example, while base stations used in a mobile telephone system are only assigned a limited number of frequencies in advance, there has been no technology for making efficient use of such communication resources.
In view of the stated problems, it is a primary object of the present invention to provide an adaptive array apparatus that can efficiently use a plurality of frequencies and can efficiently accommodate a large number of mobile devices.
The stated primary object can be achieved by an adaptive array apparatus that has a plurality of antennas, including: a first forming unit for forming an adaptive array pattern using every antenna in the plurality of antennas; a second forming unit for dividing the plurality of antennas into a plurality of groups that each include at least two antennas and forming separate adaptive array patterns for each group of antennas using a different frequency for each group; and a control unit for selectively switching between pattern formation by the first forming unit and pattern formation by the second forming unit.
The present adaptive array apparatus switches between a first operation mode where path multiplexing is performed with an array antenna formed of all the antennas and a second operation mode where path multiplexing is performed with array antennas formed of the antennas split into groups. By switching as required between the operation modes, the adaptive array apparatus can switch between using one frequency or a plurality of frequencies. This results in the adaptive array apparatus efficiently using frequencies and enables the adaptive array apparatus to efficiently accommodate a large number of mobile devices.
Here, the adaptive array apparatus may be used as a base station for mobile telephones that use a time division multiple access (TDMA) method, and the control unit may switch between pattern formation by the first forming unit and pattern formation by the second forming unit according to the number of telephones to be connected.
Also, the second forming unit may not form an adaptive array pattern for one of the groups of antennas during a time slot that is used as a control channel and may form adaptive array patterns for other time slots.
In addition to the effects described above, the present adaptive array apparatus can be used as a radio base station. Since there will be a control channel that is not suited to path multiplexing, a larger number of communication channels can be obtained by using the second operation mode (group mode) instead of the first operation mode (all-antenna mode).
Here, the number of antennas may be m*n (where m and n are integers that are no less than two), and the second forming unit may divide the plurality of antennas into n groups of m antennas.
With the stated construction, each group includes an equal number of antennas, so that by setting the number of signals that are subjected to path multiplexing at an equal value for each group, the formation of adaptive array patterns can be simplified.
The adaptive array apparatus may further include a digital signal processor, the first forming unit may have the digital signal processor calculate gain values and phase amounts of transmission and reception signals of each antenna in the plurality of antennas and use the calculated gain values and phase amounts to form an adaptive array pattern, while the second forming unit may have the digital signal processor calculate gain values and phase amounts of transmission and reception signals of antennas separately for each group and use the calculated gain values and phase amounts to form separate adaptive array patterns for each group.
The present adaptive array apparatus forms adaptive array patterns by executing programs using a digital signal processor. This facilitates the setting of the number of groups, the number of antennas, and the number of antennas in each group at desired values and the dynamic adjustment of such values.